I might be missing something really obvious, but I’ve downloaded both the iOS and MacOS distributions and I can not find the API documentation or a general usage guide anywhere. I have the examples, I can get them to work, but I need more information on when/how to use channels, sound groups and channel groups.

I actually just ran into the same thing. After a short bit of poking around on google, turns out that the "fmodex.chm" file is the documentation (chm is compiled html). A quick search of sourceforge came up with this project:

[url:mhha3xdx]http://chmox.sourceforge.net/[/url:mhha3xdx].
Works great for me, just opened the docs with it.

I’m glad you discovered our CHM documentation. We are aware how obscure this is for people coming from a Mac or Linux background and are planning to address it soon in the form of a more accessible documentation format.